1. Field of the Invention
The invention relates to the preparation of acrylic resins having elastic properties and more particularly to the preparation of ionically cross-linked acrylic resins having elastic properties.
2. Description of the Prior Art
Synthetic resins are known to exhibit elastic properties when they are cross-linked and have a low glass transition temperature. Covalently cross-linked synthetic resins have no thermoplastic region and cannot be plastically deformed. On the contrary, ionic cross-linking is reversed upon heating and permits thermoplastic deformation.
Accordingly, it remains an objective to prepare ionically cross-linked acrylic polymers having low glass transition temperatures in powder form by a process which is technically easy to carry out.
Several multi-step processes are known for preparing ionically cross-linked acrylic polymers. In these processes an uncross-linked polymer containing carboxyl groups is first prepared, and this polymer is ionically cross-linked in a second step with a metal compound. According to U.S. Pat. No. 2,726,230, a polymer containing carboxyl groups is mixed with an oxide of a multivalent metal at elevated temperature, and heated to temperatures of 50.degree. to 200.degree. C. for cross-linking. In a process described by W. Cooper in Journal of Polymer Science, Vol. 28 (1958), pp. 195-206, this conversion is carried out in solution in an organic solvent, and the polymers cross-linked with metal are precipitated by adding a non-solvent. The object of U.S. Pat. No. 3,493,550 is an improvement of this process. A solution of a polymer containing carboxyl groups which is immiscible with water is brought into contact with an aqueous solution of a metal salt. An ionically cross-linked polymer is formed at the phase boundary and is separated in granular form.
The disadvantage of these known processes lies, on the one hand, in the multi-step, and consequently expensive, procedure, and on the other hand in the insufficient uniformity of the cross-linking. Since in all cases the metal compound acts on the already formed polymer, the cross-linking occurs at the points of contact between the polymer and the metal salt. Since a highly uniform cross-linking combined with a limited density of cross-linking is required for the preparation of elastomers, the preparation of ionically cross-linked acrylic elastomers by such a two-step process has not been reported.
A one-step process is disclosed in German OS No. 27 26 260 in which an ionically cross-linked block copolymer is obtained by polymerizing a mixture of acrylic or methacrylic acid with acrylonitrile or methacrylonitrile and a metal salt of acrylic or methacrylic acid in the absence of solvent. The highly polar monomer mixture of these acids and nitriles provides a good solvent for various metal salts, but leads to a polymeer having a very high glass transition temperature which is devoid of elastic properties at room temperature.
By the process of U.S. Pat. No. 3,705,137 a pulverulent, ionically cross-linked acrylic polymer is obtained by precipitation polymerization of a mixture of monomers of polyvalent metallic salts of acrylic or methacrylic acids and esters of acrylic of methacrylic acids. The precipitation polymerization is carried out in an aqueous-alcoholic medium containing 5 to 50% of water. This water content is indicated as indispensible in order to solubilize the salt-like cross-linker which is used in a proportion of 8 to 75 mole %, calculated on the equivalents of unsaturated acid residues. Although the basic process also permits the preparation of relatively weakly cross-linked acrylic polymers with low glass transition temperatures, all the examples are concerned only with the preparation of highly cross-linked precipitation polymers devoid of elastic properties.
In order to prepare ionically cross-linked acrylic elastomers by this process, the monomer composition must be so chosen that the glass transition temperature does not exceed 30.degree. C., preferably 15.degree. C., and the proportion of ionic cross-linking monomer components does not exceed 30% by weight. The metal salts of acrylic or methacrylic acids with polyvalent metal cations which are used for cross-linking are prone to hydrolysis. Consequently, they are partially hydrolyzed by the water present in the reaction mixture and in turn catalyze the hydrolysis of the acrylic ester. Both the resulting metal hydroxides and the free acrylic or methacrylic acids increase the glass transition temperature and decrease the elastic properties. Accordingly, ionically cross-linked acrylic elastomers having a sufficiently uniform cross-linking cannot be prepared by the known processes.
Hence a need has continued to exist for amethod of preparing an ionically cross-linked acrylic resin having elastic properties.